[op25-dev] Re: Full asterisk support?

ikjtel ikj1234i at yahoo.com
Sun Feb 24 21:08:34 UTC 2013

> Thinking about this some more, why couldn't you just split the audio from the RX and TX into two

Yeah, asterisk app_rpt transmitting analog FM voice over standard FM radio hardware is already a fully solved problem having no need for a USRP.
Our software stack can receive analog NBFM in addition to P25, for one thing because the software support to do so was not difficult to add.
> I thought the USRP_TX app was only for that hardware. I want to use a conventional radio, just as you described, for both P25 and NBFM. So sign me up as as experimenter wanting to try that!
OK -excellent.  For starters could we try first to get python/op25_audio_tx.py working - that app does not support asterisk / app_rpt but will establish a baseline.  The results of that test would direct the next step of the experiment.  

op25_audio_tx takes input analog voice either from a sound card or from a disk file (8,000 samples/sec, short signed int16 format).  It generates audio at the soundcard audio line-out port (which should be connected directly to the modulator stage of the FM TX).  You might be able to use the same sound card for both lines, if the card supports full duplex and if it allows different rates for record and playback.  If not the next best thing might be to use canned test audio files [use the -i option and place the audio test file in baseband-0.dat].

The biggest problem so far has been group delay distortion.  It's also critical to get the proper audio signal levels going into the TX such that the FM deviation is correct.
That should be it - let me know of next step(s)


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